See:

https://i.imgur.com/ZphvVST.png

This is done when someone passes who is considered especially (noteworthy|significant|involved|something) to/with the HN community. AFAIK there's no exact formula for determining who the mods deem worthy of that tribute, but I do know that they don't do it for everybody famous who dies. It's usually people who are particularly close to the HN world in some way.

I believe that the black bar is put up at the top of the site after someone who is considered important by HN readership dies.